1. Field of the Invention
The present invention relates to an excavating, sieving and grading device mounted as an attachment on a working crawler such as a hydraulic shovel as an example of construction machines, an agricultural tractor as an example of agricultural machines, and the like.
2. Description of the Related Art
In general, an excavating, sieving and grading device, which has an excavating function for excavating mud, grit, soil broken in a construction field, and the like and a sieving and grading function for sieving and grading the excavated material, is mounted on a working crawler such as a hydraulic shovel and the like as an attachment. Such an excavating, sieving and grading device is known from Japanese Utility Model Application Laid-Open No. 2(1990)-133481 and Japanese Utility Model Application Laid-Open No. 3(1991)-125850. The excavating, sieving and grading devices disclosed in these publications are arranged such that the bottom surface of a bucket main body mounted on an arm and having an excavating function is opened and a movable frame which is provided with a sieve net and reciprocatingly movable in a horizontal direction is attached to the opening, so that excavated material is sieved and graded by the horizontal reciprocating movement of the movable frame.
The movable frame of these excavating, sieving and grading devices is reciprocatingly moved in such a manner that guide rails integrally provided with the both right and left sides of the movable frame are supported and guided by guide rollers disposed on the both right and left side walls of the bucket main body.
Although the movable frame must be so strong as to endure the total weight of an excavated material placed on the sieve net, when the movable frame is supported only on the both right and left edges thereof, the weight of the frame cannot help being increased to provide the frame with a strength sufficient to prevent the downward deflection of the central portion of the frame. Thus, a large drive unit corresponding to the heavy movable unit is required to drive the movable frame.
Moreover, although a countermeasure for sufficiently increasing the strength of the bucket main body which supports the heavy movable frame is required, since it is indispensable to arrange the bucket main body as a bottomless structure with the opening defined to the bottom surface thereof, an employable countermeasure is only to increase the wall thickness of the side walls of the bucket main body. With such a countermeasure for increasing the strength, however, the wall thickness must be considerably increased to enable the bucket main body to support the movable frame which moves in the horizontal direction, and thus there arises a problem that the size and weight of the excavating, sieving and grading device are more increased as a whole.
Further, the aforesaid conventional excavating, sieving and grading device also has a problem that although the sieve net sieves and grades excavated material by moving in the horizontal direction forwardly and backwardly, when the excavated material is wet, the material adheres to the meshes of the sieve net and is liable to cause clogging. When clogging is caused, a sieving and grading function is lowered and thus workability is greatly lowered.